Over the past decade, the development of cleavable surfactants has been a growing field in surfactant science. As the name implies, cleavable surfactants are molecules that undergo a chemical or physical change of the parent molecular structure resulting in a change and/or loss of surface-active behavior. Hence, the production of commercially available cleavable surfactants would find utility in industrial practices where foaming or persistent surface-active properties must be diminished after their initial use, in “green” chemistry where biodegradability is of primary concern, and in biomedical drug delivery where surfactants could be removed through biological mechanisms.
Additionally, surfactant removal becomes increasingly significant in the synthesis of extended mesoporous and nanosized structures such as semiconductor nanocrytsals, ceramics, polymers, and polymer-ceramic composites. The current techniques of surfactant removal are typically a combination of centrifugation, calcination, and solvent washing steps that can adversely affect and/or completely destroy the desired extended architecture and functionality of the synthesized material. Incorporation of a cleavable linkage into surfactant molecules could solve this problem by allowing the removal of the surfactant templates through the thermally keyed formation of small, easily removed fragments.
Several examples of cleavable surfactants have been previously reported based on functional groups that are susceptible to alkaline or acid hydrolysis. The surfactants operate at set pH ranges and are removed from the system by adding an appropriate amount of acid or base. Acid-labile surfactants include cationic surfactants with cyclic acetals such as cationic surfactants derived from bromopropionaldehyde, alkylglucosides, ortho esters, and both cyclic and noncyclic ketals (see for instance Wang, G.-W., et al., Am. Oil Chemists Soc. 1995, 72, p 83; Rybinski, W.-V., Curr. Opin. Colloid Interface Sci. 1996, 1, p 572; Eliason, R., et al., J. Am. Chem. Soc. 1978, 100, p 7037; Jaeger, D. A., et al., J. Org. Chem. 1993, 58, p 2619; and Ono, D., et al., J. Org. Chem. 1990, 55, p. 4461). Alkaline-labile surfactants include surfactants that contain cleavable ester moieties such as choline esters, esters of quaternized amidoamines and ethanolamines, as well as esters derived from naturally occurring sugars (i.e., glucose, sucrose, sorbitol) combined with fatty acids (see for instances Ahlstrom, B., et al., Antimicrob. Agents Chemother. 1995, 39, p 50; Lagerman, R., et al., J. Am. Oil Chemists Soc. 1994, 71, p 97; Swartley, D. M., et al., U.S. Pat. No. 5,399,272; and Ducret, A., et al., J. Am. Oil Chemists Soc. 1996, 73, p 109). However, hydrolysable surfactants are disadvantaged and their utility limited due to the requisite addition of acid or base to degrade the surfactant, or in those applications where a neutral pH is required.
Other examples of cleavable surfactants wherein labile elements that have been incorporated into the surfactant molecule include UV sensitive components, such as alkylarylketone sulfonates and diazosulfonates which undergo surfactant cleavage upon irradiation (see Epstein, W. W., et al., Anal. Biochem. 1982, 119, p 304; and Nuyken, O., et al., J. Photochem. Photobiol. A Chem, 1995, 85, p 291), and some examples of amine oxide surfactants which decompose at temperatures above 100° C. (see Hayashi, Y., et al., J. Am. Oil Chemists Soc. 1985, 62, p 555).
To overcome these shortcomings, we describe here the synthesis and characterization of two new surfactant compositions which incorporate a thermally cleavable Diels-Alder adduct as the chemical weak link within the surfactant molecular structure. In particular, we have utilized the reversible Diels-Alder reaction between functionalized furans and maleimides as the basis for a thermally cleavable material. We have previously reported the integration of furan-maleimide Diels-Alder adducts into molecules to produce thermally responsive encapsulating polymers, foams, and adhesives as well as dendrons and dendrimers which reversibly self-assemble (see U.S. Pat. Nos. 6,271,335; 6,337,384; and 6,403,753 and McElhanon, J. R., et al., J. Appl. Pol. Sci. 2002, 85, p. 1496; Aubert, J. H., Journal of Adhesion 2003, 79, pp. 609-616; and Aubert, J. H, U.S. Published Application No. 20030116272; McElhanon, J. R., et al., Org. Lett. 2001, 3(17), p. 2681). Also, similar thermally reversible Diels-Alder adducts have been reported incorporated into other responsive polymers (see Chen, X., et al., Macromolecules 2003, 36, p 1802-1807; and Chen, X., et al., Science 2002, 295, p 1698-1702).